Build a few homes and you will run into this same problem sooner or later.
Running straight up and out may be the shortest distance but during cooler seasons, as the warm and steamy air moves up the exhaust duct, it will cool and the condensation will run back down the duct and could appear as a leak. If you go up into the attic and across, you have an easy opportunity to lay insulation directly on the duct and reduce (not eliminate) the condensation. But at 20' you will certainly still have the potential for a moisture problem.
Either way you go, Do seal the exhaust duct to the fan box and all joints in whatever type of ducting you use with a good quality foil tape. At least then, when/if the moisture problems surface, they will be manageable without destroying ceilings or worse.
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